1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same and, particularly, to a fringe-field switching mode liquid crystal display device and a method of manufacturing the same.
2. Description of Related Art
An in-plane switching (IPS) mode of a liquid crystal display device is a display technique that displays an image by applying an in-plane electric field to liquid crystal placed between substrates. The IPS mode provides better viewing angle characteristics than a twisted nematic (TN) mode, and it is expected to meet the demand for high quality pictures.
An IPS mode liquid crystal display device generally has a structure in which a pixel electrode and a counter electrode are formed by metal films and arranged opposite to each other on the same substrate. In the liquid crystal display device having such a structure, it is difficult to increase a pixel aperture ratio compared with the TN mode, thus having low light use efficiency.
In order to improve the aperture ratio and the transmittance in the IPS mode liquid crystal display device, a fringe-field switching (FFS) mode has been proposed (e.g. Japanese Unexamined Patent Applications Nos. 2001-235763 and 2002-182230). An FFS mode of a liquid crystal display device is a display technique that displays an image by applying a fringe electric field to liquid crystal placed between substrates. Because a pixel electrode and a counter electrode are formed by transparent conductive layers in the FFS mode liquid crystal display device, the aperture ratio and the transmittance are higher than those of the IPS mode. Further, because capacitance is formed between the transparent conductive layers in the FFS mode liquid crystal display device, there is no loss of transmittance due to a capacitance forming portion.
In the FFS mode liquid crystal display device according to related art, liquid crystal is driven by a fringe electric field that is generated between the pixel electrode having a slit placed in an upper layer and the counter electrode placed in a lower layer with an insulating layer interposed therebetween. The pixel electrode is placed away from the source line in each pixel so as not to overlap the source line in order to reduce the capacitance between the source line and the pixel electrode. Specifically, the pixel electrode is placed at a certain distance from the source line. By reducing the capacitance between the source line and the pixel electrode, it is possible to prevent deterioration of display quality.
In this structure, however, when a voltage is applied to the source line, an electric field is generated by the voltage, causing a change in the orientation of liquid crystal over a relatively wide range in the vicinity of the source line. Because the counter electrode is placed in a layer that is lower than the source line in the FFS mode liquid crystal display device according to related art, the electric field from the source line cannot be shielded. As a result, light leakage occurs in the vicinity of the source line. In order to block the leakage light in the vicinity of the source line, a black matrix to cover the source line and the vicinity of the source line is placed on the counter substrate side in the FFS mode liquid crystal display device according to related art. The black matrix is placed to overlap the source line and the relatively wide range in the vicinity of the source line. Thus, an invalid region (non-transmitting region) that does not contribute to display increases in the vicinity of the source line, causing a decrease in aperture ratio.
Further, in the FFS mode liquid crystal display device according to related art, the pixel electrode is placed in a separate layer from a drain electrode of a thin film transistor with an insulating layer interposed therebetween. In this structure, it is necessary to form a contact hole for electrically connecting the pixel electrode and the drain electrode. Accordingly, the structure needs to have the size necessary for forming the contact hole and a positional allowance around the contact hole. Because the region is a non-transmitting region that does not allow transmission of light, this is one cause of a further decrease in aperture ratio.
In light of the foregoing, it is desirable to provide an FFS mode liquid crystal display device capable of improving an aperture ratio and a method of manufacturing the same.